Man-made satellites orbiting around the earth are typically categorized as being in one of a geosynchronous earth orbit (GEO), a medium earth orbit (MEO), a low earth orbit (LEO), or a highly elliptical earth orbit (HEO). Each of these types of orbits has particular advantages and disadvantages, which take on relatively different importance depending on the application for which the satellite is being used.
GEOs have the advantage that the satellite orbits at generally the same angular rate around the earth as the earth is rotating (approximately one orbit every 24 hours), so that the satellite appears to hover over a given ground point on the earth. This is achieved in part by having the satellite orbit near the earth's equator and being approximately 36,000 km above the earth. The GEO satellite has access a large area on the earth, due to its high altitude. This area is known as the field of regard (FOR). One disadvantage of GEO satellites is that it is much more expensive to launch the satellite to the necessary GEO altitude as compared to satellites at lower altitudes. Another disadvantage is that a sensor deployed on a GEO satellite would have to be more capable than its LEO counterpart to maintain the desired accuracies over the longer ranges.
LEO satellites generally reside less than about 2500 km above the earth, and have an orbital period of 1.5 to 2 hours and can fly in either circular or elliptical orbits around the earth. LEO satellites generally have opposite advantages and disadvantages as compared to GEO satellites. An LEO satellite can more easily obtain higher resolution due to its closer proximity to the earth, and is less expensive to launch for the same reason. One of the strong characteristics of LEO satellites is that they are not stationary relative to ground points. This characteristic has several implications, including the fact that a given satellite can only access a given point on the earth for a limited time duration (known as the access time). In addition, there is a gap in time until the satellite can see the same ground point again (known as the gap time). Further, LEO satellites have a much smaller FOR than GEO satellites.
MEO satellites have less extreme advantages and disadvantages than the GEO and LEO satellites described above, since they are at an intermediate altitude. MEO satellites generally are located between LEO and GEO altitudes and have an orbital period of 2-24 hours. These satellites have larger FOR and longer access times than LEO satellites, with higher resolution and more power and antenna requirements, and are less expensive to launch than GEO satellites.
Highly eccentric orbits (HEO) typically have perigee altitudes in the LEO regime and apogee altitudes above GEO ranges. Due to the wide variations in altitude as the satellite moves from its lowest altitude (perigee) to its highest altitude (apogee), its sensors are typically designed to operate about the apogee position (where the satellite spends most of its time). This type of orbit can provide long periods of local access at higher latitudes than are normally achieved from GEO.
Most work to date with satellite constellations has related to providing continuous access from the satellite constellation to a given ground point. In different approaches, this may include global access, polar-cap access, or zonal access within specific latitude bands. Providing continuous access can be very expensive, however, as many satellites can be required. The exact number of satellites required is a function of the altitude of the satellites and latitude of the area for which continuous access is required. For example, since a GEO satellite may have an FOR that covers 40% of the earth's surface, three different GEO satellites spaced 120° apart from the earth's equator can together see a large percentage of the earth's surface. Most importantly, the typical approaches for providing continuous access use an equally-spaced, symmetric pattern around the earth.
Because the number of LEO satellites required to provide continuous access to a given ground spot are so high, satellite constellations designed for LEO satellites are often focused on reducing the gap time between when sequential satellites can see a given ground point. Unfortunately, by distributing a limited number of satellites in a symmetric pattern to minimize gap times, access is limited to the duration that a single satellite can see a given ground point. While this approach may be desirable in many applications, there are other applications in which increasing the contiguous access time is an important objective. For example, in a low earth orbit, a typical satellite may only have access to a given ground point for a 15-minute period of time. Therefore, if it is desired to continuously monitor a given ground point for a longer period of time (for example, 30 minutes), it may not be possible unless an entirely populated, symmetrical constellation is provided with a great number of satellites to provide continuous access. In addition to a satellite having extended access to a ground target, it may also be desirable for a given satellite to have an extended duration of communication to a ground-based communication site.
Another type of satellite grouping is known as a cluster. Satellite clusters typically include a plurality of satellites traveling together in close proximity within the same or very closely spaced orbit plane. The FOR of each of the satellites may almost entirely overlap each other. Clusters may be used so that each of the satellites in the cluster can have a different function or capability, for example.
It is against this background and with a desire to improve on the prior art that the present invention has been developed.